Venturi Fertilizer Injectors
Venturi fertilizer injectors are a great way to mix fertilizer into your irrigation system. They have no moving parts, require no electricity to operate, and are practically maintenance free. They use your irrigation system's water flow to create suction that draws liquid fertilizer into the system.
The Right Size
It is very important to choose the right size injector for your irrigation system or it may not create enough suction to draw any fertilizer at all. Before you purchase one, you must know the flow rate of your irrigation system so you can match it with the right size injector. If you go too big, your injector will not draw fertilizer. If you go too small, you will be restricting your system's flow.
How They Work
A pressure differential between the inlet and outlet of a Venturi injector creates a vacuum that causes suction at the suction port. 
Pressurized water flows through the inlet of the Venturi injector and into a constricted channel where its velocity increases and its pressure drops. This drop in pressure creates a vacuum at the suction port.
A small pressure differential (about 25%) between the inlet and outlet will create a slight vacuum and therefore a less suction. If the pressure differential is too low the suction will not be strong enough to draw fertilizer.
A larger pressure differential will create a more powerful vacuum and greater suction of fertilizer. Maximum suction occurs around 50% pressure difference.
Installation and Plumbing
Venturi fertilizer injectors should be installed on a bypass assembly for less restriction and greater control of adjustment. They can also be plumbed directly into a mainline, however, this method is not recommended and may not work with all irrigation systems.
A Venturi injector's outlet should remain flooded. Some backpressure is required for it to work properly. Vertical installation of an injector with inlet on top and outlet below (water flowing downward) is improper and will result in erratic or no suction.
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Troubleshooting
If your Venturi fertilizer injector is not drawing fertilizer, it may be due to one or more of these reasons:
- No vacuum. Your injector must be completely airtight to create a vacuum strong enough to suck fertilizer up the suction tube. Make sure you have a rubber gasket between all connections on the injector and rotameter. Some Venturi injector kits come partially unassembled and may have a loose gasket in its poly bag packaging or a gasket may have fallen out of its seat.
- Not enough water flow – also called motive flow – to create the pressure differential needed to create sufficient suction. Refer to the performance chart to see what kind of motive flow and inlet/outlet pressure is needed for your injector model. You may need to switch to a smaller injector and/or use a bypass setup.
- Fertilizer container is positioned too low relative to Venturi injector. Try elevating the fertilizer container so that less vacuum is required to lift the fertilizer solution up the suction tube.
- Throttle valve on suction hose is in closed position. Make sure the valve on the suction hose is open.
Performance Data
Motive flow refers to the irrigation water flowing through the Venturi injector while water suction refers to the flow of fertilizer solution being sucked into the system. Inlet pressure is your starting pressure. Outlet pressure is zero when there is no backpressure on the system such as an open discharge hose or pipe.
